For data or command signal transmission, transmission antennas must be capable of irradiating or radiating one or more electrical signals with minimum losses and with a properly matched bandwidth. Thus, all the spectral components of the electrical signals are radiated in the best possible manner.
For small electrical antennas, namely antennas which have a much smaller physical size than the transmission signal wavelength, it is necessary to define a compromise between power loss and the bandwidth of the signals to be transmitted. As a general rule, for small antennas with a reasonable bandwidth, the product between power loss and signal bandwidth must be constant. This property may also be explained by the quality factor expression Q.
Some progress has already been made towards producing antennas beyond this operating limit, but without attaining the hoped-for operation. Thus, transmission antennas do not exist which are capable of radiating electrical signals with a good quality factor Q taking into account the Chu limit. The Chu limit for conventional antennas describes the minimum quality factor, and consequently the maximum bandwidth, according to the size of the conventional antenna. The Chu limit, well known for developing said antennas, is based on the hypothesis that said antenna is passive, linear and small.
To precisely determine this quality factor Q, reference may be made to the article entitled “A re-examination of the fundamental limits on the radiation Q of electrically small antennas” by J. S. McLean in the journal IEEE transaction on antennas and propagation, 1996, vol. 44, issue 5. Some small antenna embodiments are also described in the article entitled “Electrically small super-directive and super-conductive antennas” by R. C. Hansen in the 2006 work by Wiley, pages 62 to 84.